linux-kernel - Re: [PATCH v3 1/2] usbip: Skip DMA mapping and unmapping for urb at vhci

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Date:   Mon, 5 Aug 2019 09:08:18 -0600
From:   shuah <shuah@...nel.org>
To:     Suwan Kim <suwan.kim027@...il.com>
Cc:     valentina.manea.m@...il.com, gregkh@...uxfoundation.org,
        stern@...land.harvard.edu, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, shuah <shuah@...nel.org>
Subject: Re: [PATCH v3 1/2] usbip: Skip DMA mapping and unmapping for urb at
 vhci

On 8/4/19 11:23 PM, Suwan Kim wrote:
> On Fri, Aug 02, 2019 at 04:22:27PM -0600, shuah wrote:
>> On 8/2/19 11:36 AM, Suwan Kim wrote:
>>> vhci doesn’t do DMA for remote device. Actually, the real DMA
>>> operation is done by network card driver. vhci just passes virtual
>>> address of the buffer to the network stack, so vhci doesn’t use and
>>> need dma address of the buffer of the URB.
>>>
>>> But HCD provides DMA mapping and unmapping function by default.
>>> Moreover, it causes unnecessary DMA mapping and unmapping which
>>> will be done again at the NIC driver and it wastes CPU cycles.
>>> So, implement map_urb_for_dma and unmap_urb_for_dma function for
>>> vhci in order to skip the DMA mapping and unmapping procedure.
>>>
>>> When it comes to supporting SG for vhci, it is useful to use native
>>> SG list (urb->num_sgs) instead of mapped SG list because DMA mapping
>>> fnuction can adjust the number of SG list (urb->num_mapped_sgs).
>>> And vhci_map_urb_for_dma() prevents isoc pipe from using SG as
>>> hcd_map_urb_for_dma() does.
>>>
>>> Signed-off-by: Suwan Kim <suwan.kim027@...il.com>
>>> ---
>>>    drivers/usb/usbip/vhci_hcd.c | 19 +++++++++++++++++++
>>>    1 file changed, 19 insertions(+)
>>>
>>> diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
>>> index 000ab7225717..c62f7fa8118c 100644
>>> --- a/drivers/usb/usbip/vhci_hcd.c
>>> +++ b/drivers/usb/usbip/vhci_hcd.c
>>> @@ -1288,6 +1288,22 @@ static int vhci_free_streams(struct usb_hcd *hcd, struct usb_device *udev,
>>>    	return 0;
>>>    }
>>> +static int vhci_map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
>>> +		gfp_t mem_flags)
>>> +{
>>> +	if (usb_endpoint_xfer_isoc(&urb->ep->desc) && urb->num_sgs) {
>>> +		WARN_ON(1);
>>
>> Don't add WARN_ON. I cleaned them all up recently and don't want new
>> ones added.
> 
> Ok. I will remove it and resend v4.
> 

Please add appropriate error message in place of WARN_ON

thanks,
-- Shuah